Nested loop execution in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested loop in PHP?

A nested loop is a loop inside another loop. The inner loop runs completely every time the outer loop runs once.

beginner

How many times will the inner loop run in this code?<br>

<?php
for ($i = 1; $i <= 3; $i++) {
  for ($j = 1; $j <= 2; $j++) {
    echo "$i,$j ";
  }
}

The inner loop runs 2 times for each of the 3 outer loop runs, so 3 × 2 = 6 times total.

beginner

What is the output of this nested loop?<br>

<?php
for ($i = 1; $i <= 2; $i++) {
  for ($j = 1; $j <= 3; $j++) {
    echo $i * $j . ' ';
  }
}

The output is: 1 2 3 2 4 6 <br>Explanation: For i=1, j=1 to 3 outputs 1,2,3. For i=2, j=1 to 3 outputs 2,4,6.

intermediate

Why do nested loops often have higher time complexity?

Because the inner loop runs completely for each iteration of the outer loop, the total steps multiply, making the program slower as input grows.

beginner

How can you visualize nested loops in real life?

Imagine you have 3 boxes, and inside each box are 2 balls. You open each box (outer loop) and then pick each ball inside (inner loop).

In PHP, if the outer loop runs 4 times and the inner loop runs 5 times, how many times does the inner loop execute in total?

A 20

B 9

C 5

D 4

The inner loop runs 5 times for each of the 4 outer loop runs, so 4 × 5 = 20 times.

What will this PHP code output?<br>

<?php
for ($i=1; $i<=2; $i++) {
  for ($j=1; $j<=2; $j++) {
    echo $i . $j . ' ';
  }
}

A 1 2 2 2

B 11 12 21 22

C 12 21 22 11

D Error

The output is '11 12 21 22 ' because for i=1, j=1 and 2 prints 11 and 12; for i=2, j=1 and 2 prints 21 and 22.

Which statement is true about nested loops?

A Outer loop runs inside inner loop

B Inner loop runs once for all outer loop iterations

C Inner loop runs completely for each outer loop iteration

D Nested loops cannot be used in PHP

The inner loop runs fully every time the outer loop runs once.

What is the time complexity of two nested loops each running n times?

A O(log n)

B O(n)

C O(1)

D O(n^2)

Two nested loops each running n times result in O(n × n) = O(n^2) time complexity.

In PHP, which keyword is used to start a loop?

A for

B loop

C repeat

D iterate

The 'for' keyword is used to start a for loop in PHP.

Explain how nested loops work in PHP with an example.

Think about a loop inside another loop and how many times the inner loop runs.

Describe a real-life situation that helps you understand nested loops.

Think about opening boxes and picking items inside.